Pipe diameter and pipe dimensions – DN, d, wall thickness e and pipe size table. A practical guide

Selecting the right components for a piping system involves more than comparing two numbers in a catalogue. Markings such as DN, d, e, mm and SDR refer to different parameters, and their meaning depends on the material, standard and intended application. Steel pipe, PVC-U pipe and PE pipe may all be specified differently, which means that the same numerical value does not always represent the same dimension.

In this article, we explain how to read pipe dimensions, the difference between DN and d, what wall thickness e means, and why the outside diameter alone is not enough to fully define a pipe. We also explain SDR and show how to interpret product data when selecting pipes, fittings and valves.

This information is useful for designers, installers, distributors and technical purchasing teams. A correctly specified pipe simplifies component selection, reduces the risk of ordering errors and makes it easier to complete an entire piping system.

What is pipe diameter and which dimensions matter?

The question may seem straightforward, but several different dimensions need to be distinguished in piping technology. A pipe has an outside diameter, an actual inside diameter and a specific wall thickness. Technical documentation may also refer to the nominal diameter DN, which serves a different purpose from a physical dimension measured directly on the pipe.

The most important markings can be summarised as follows:

For a pipe with a uniform wall thickness, the inside diameter can be approximated using the following formula:

This means that two pipes with the same outside diameter can have different inside diameters if their wall thicknesses differ. Even a difference of a few millimetres can affect flow, pressure loss, pipe weight and operating characteristics.

The diagram shows the relationship between the main dimensions of a pipe cross-section. d represents the outside diameter, d_i the actual inside diameter and e the wall thickness.

What does nominal diameter DN mean?

DN is a nominal designation used to identify the size of components within a piping system. ISO 6708 defines DN as a nominal size designation. The number following DN should not automatically be interpreted as an outside or inside diameter measured in millimetres.

For example, DN20 does not mean that every pipe has an exact inside diameter of 20 mm. The actual dimensions depend on the piping system, material, dimensional series and wall thickness. In English-language technical documentation, terms such as nominal size and nominal diameter are commonly used.

DN is particularly useful when specifying valves and other piping components. A valve, flange, butterfly valve or other component may be assigned a DN value to indicate the nominal system size for which it is intended. However, the actual connection dimensions still need to be checked.

Important: the illustration shows DN schematically. DN is not a direct measurement of the pipe's inside diameter. A physical measurement of the internal bore gives the inside diameter d_i, not the DN value.

DN versus d – what is the difference?

DN and d describe different parameters. DN is a nominal system designation, whereas in many metric piping systems the symbol d refers to the outside diameter.

For plastic pipes, the outside diameter is often one of the main dimensions used for product identification. A d63 pipe has a nominal outside diameter of 63 mm. This does not mean that its inside diameter is also 63 mm, because the wall thickness must be taken into account.

Bosta offers PVC-U pipes with the same outside diameter but different wall thicknesses. This clearly demonstrates why the d dimension alone does not fully define the geometry of a pipe.

What does e mean and why is pipe wall thickness important?

The symbol e represents the wall thickness of a pipe. It is the distance between the external and internal surfaces of the pipe wall. This dimension directly affects the inside diameter and therefore the available flow area.

If two pipes both have an outside diameter of d63, but one has a wall thickness of 2.4 mm and the other 4.7 mm, their internal bores will be noticeably different.

The table illustrates why the same outside diameter does not necessarily mean the same internal bore. As wall thickness increases while d remains unchanged, the actual inside diameter decreases.

How do you read a pipe size such as 63 × 4.7 mm?

A designation such as 63 × 4.7 mm provides considerably more information than simply stating “63 mm”. In this example, the first figure refers to the outside diameter d, while the second indicates the wall thickness e.

This is why professional product selection should not be based solely on a request such as “I need a 63 mm pipe”. The material, diameter, wall thickness or SDR series, pressure class and connection method should all be specified.

How should steel pipe dimensions be read?

Steel pipe dimensions are often specified using DN together with actual geometric dimensions defined by a particular dimensional series. For steel pipes, the nominal DN value should not be treated as the measured outside diameter.

A DN20 pipe does not necessarily have an outside diameter of 20 mm. Likewise, a 22 mm pipe is not automatically equivalent to DN20. Standard steel pipe sizes must be interpreted according to the standard under which the particular product has been manufactured.

Wall thickness is also important. A steel pipeline with the same outside diameter may be available in several wall thicknesses. Changing the wall thickness affects the inside diameter, weight and mechanical properties.

When a steel component is connected to a plastic system, comparing DN and d alone is not enough. The correct adaptor, fitting or transition component must be selected for both systems.

How are plastic pipe diameters specified?

In plastic piping systems, the nominal outside diameter d is often one of the primary dimensions used to identify a pipe. For industrial ABS, PVC-U and PVC-C systems, DIN EN ISO 15493 is one of the relevant reference standards for metric industrial piping systems.

If a pipe is marked d20, the value 20 mm refers to its nominal outside diameter. Similarly, a reducer described as d90 to d63 connects two different nominal outside diameters.

For solvent-cemented systems, matching the pipe dimensions correctly to the fitting socket is essential for a reliable connection. See the PVC-U pipes and fittings available from Bosta .

What is SDR and how does it relate to wall thickness?

SDR stands for Standard Dimension Ratio. It expresses the relationship between the nominal outside diameter of a pipe and its nominal wall thickness.

SDR values are standardised, so the calculated ratio may not always be exactly the same as the stated SDR value.

For the same outside diameter, a lower SDR value indicates a relatively thicker wall, whereas a higher SDR value indicates a relatively thinner wall. In practice, SDR should always be considered together with the pipe material, operating temperature and application.

Two d63 pipes can therefore have the same outside diameter but different SDR values, wall thicknesses and inside diameters. This is why the d dimension alone is not sufficient when specifying a pipe for a technical application.

DN, mm, inches and diameter symbols – why simple conversion can be misleading

One inch is mathematically equal to 25.4 mm, but this does not mean that every nominal pipe size can be converted directly using this value.

DN is not a unit of length. It identifies a nominal component size, whereas millimetres describe an actual geometric dimension. Diameter symbols such as Ø are also widely used, but without additional context they do not necessarily indicate whether the dimension is external, internal or nominal.

If technical documentation gives a value as d, DN, Ø or in inches, it is important to check which system the designation refers to. For designers and purchasing teams, comparing the full specification is safer than relying on a universal conversion chart.

How do you measure pipe diameter and wall thickness?

If the manufacturer's markings are no longer visible, a vernier caliper can be used as a basic measuring tool. It can measure the outside diameter and, where the pipe cross-section is accessible, the wall thickness.

To measure the outside diameter correctly, the caliper jaws should be positioned perpendicular to the pipe axis. It is advisable to take measurements at several points, particularly if the surface is dirty, coated or mechanically damaged.

However, the measured physical dimension does not automatically determine DN. If a caliper shows an outside diameter of 63 mm, this may correspond to d63 in a system that uses outside diameter for pipe sizing. It does not mean DN63.

Where the pipe cross-section is accessible, e can also be measured. Knowing d and e allows the inside diameter to be estimated using d_i ≈ d − 2e. This can help identify an existing installation, but the final selection of a replacement component should also take account of the material, standard and manufacturer's requirements.

How do you select the correct fitting for a pipe?

Matching pipes, fittings and valves requires more than checking a single dimension. A fitting must be compatible with the material, geometry, installation method and operating conditions of the piping system.

In PVC-U piping systems , solvent cement connections are commonly used, so the pipe and fitting socket must have matching dimensions. PE systems may use butt fusion, electrofusion or mechanical fittings, while steel piping may use threaded or flanged connections.

Every component should therefore be selected in accordance with the applicable standards and system documentation. Similar numerical markings alone do not guarantee compatibility. This is particularly important in water, gas and industrial applications where dimensional accuracy and system reliability are essential.

How should pipe size tables be used?

No single universal pipe size table can accurately describe every piping system. Steel, PVC-U and PE pipes are specified in different ways and therefore require different sets of dimensional information.

For plastic systems, a table showing d, e, SDR and pressure class is particularly useful. For steel pipes, DN, outside diameter, wall thickness and the relevant dimensional series may be more important.

A table structured in this way is far more useful to a designer or installer than one containing only DN and millimetre values. It makes it possible to compare not only the nominal size but also the actual geometry of the pipe and its connection method.

Pipe diameter in practice – how can specification errors be avoided?

In practice, pipe diameter should always be considered together with the function of the component. Pipes, valves, flanges, adaptors and reducers may all use different forms of dimensional designation. Where components come from different manufacturers, their mutual compatibility and applicable standards must be checked.

Nominal diameter should not be determined solely by taking a physical measurement with a caliper. ISO 6708 defines the use of DN, while industrial ABS, PVC-U and PVC-C systems may also be covered by requirements such as DIN EN ISO 15493.

For plastic pipes, d and e are equally important. Together, these dimensions define the basic geometry of the pipe cross-section. Material, SDR, pressure class, joining method and operating temperature must also be taken into account.

For contractors, distributors and technical departments, a correctly specified pipe means fewer ordering errors, fewer returns and a more efficient installation process.

What should you remember?

• DN is a nominal piping-system size designation and should not be treated as a direct measurement of the inside diameter in millimetres.
• d represents the outside diameter in systems that use this form of sizing.
• e represents the pipe wall thickness.
• For a simple, uniform pipe section, the inside diameter can be estimated as d_i ≈ d − 2e.
• Two pipes with the same d value can have different wall thicknesses and therefore different internal bores.
• SDR describes the relationship between outside diameter and wall thickness.
• DN, millimetres and inch sizes should not be converted mechanically without understanding the relevant piping system.
• Pipes and fittings should be selected according to material, dimensions, joining method and operating conditions.
• In B2B specifications, it is better to provide a complete set of data, including material, DN or d, e/SDR, PN and connection type, rather than relying on one diameter value alone.

Pipes, fittings, valves and other piping components are available in the complete Bosta product range .